The present invention relates to a technique for heating or cooling a substrate. More specifically, the present invention relates to a substrate temperature control system and a method for controlling the temperature of a substrate, which is preferably applicable to the lithography process of producing semiconductor devices and the like.
The process of producing semiconductor devices includes a process of heating and cooling substrates. In the lithography process, in particular, heating is conducted several times in repetition. The substrate heating system includes for example a spin-coater wherein a vacuum chuck mechanism is arranged on a hot plate to which a substrate is put in close contact (for example, see Japanese Patent Laid-open No. Sho 62-53773). Additionally, there are many examples of a system of the proximity mode, by which a substrate is heated while the substrate floats slightly above a hot plate (see xe2x80x9cElectronics Parts and Materialsxe2x80x9d, published by Kogyochosakai Publishing Co. Ltd., extra number, 1994, pp. 77-83). The substrate heating system is a resist baking oven for a semiconductor wafer (proximity bake unit) as shown in FIG. 13, for example. The system is used in a process after resist coating process of the substrate (semiconductor wafer), and the figure schematically shows the system. In the figure, 1 represents a semiconductor wafer with resist coated thereon. Wafer 1 is transferred on an elevator mechanism composed of lift pin 8 and actuator 9 operable for moving the lift pin upward and downward and is then mounted on small block 52 arranged on the surface of hot plate 51. The hot plate 51 inside which heater element 4 is arranged is controlled to a predetermined temperature by means of thermocouple 6 and thermoregulator 7. Through the block 52, the wafer 1 is arranged in a floating fashion about 0.1 mm apart from the hot plate 51.
Where the substrate heating system is used for post-exposure bake (referred to as xe2x80x9cPEBxe2x80x9d) process of, for example, chemical amplified resist with the exposed part extremely temperature sensitive after lithography, the temperature variation can be controlled at about xc2x10.8xc2x0 C.; by such proximity mode with an additional control mode of the gas stream above substrates, the temperature variation can be controlled at about xc2x10.3xc2x0 C.
Following the high integration tendency of semiconductor devices in recent years, however, it is demanded that the temperature variation at the PEB process should be controlled to a more stricter value. However, the conventional substrate heating system cannot satisfy such demand, disadvantageously. Furthermore, the increase of semiconductor wafer size is now under way, so that the suppression of the temperature variation is more difficult than ever.
The increase of semiconductor wafer size induces the increase of thermal capacity, so that a longer time is required for the wafer to reach a desired temperature. In other words, a new problem of the prolongation of temperature elevation time occurs. A time of about 60 seconds is required to elevate the temperature of a wafer of an 8-inch size to the objective temperature of 60xc2x0 C. to 150xc2x0 C. The increase of temperature elevation time deteriorates the throughput (production efficiency) of the production process.
Where the float distance of the substrate from the hot plate is small as described above, gaseous heat conduction according to Fourier law of heat conduction from the hot plate to the substrate is predominant while the heat transfer through gaseous convection is negligible. As will be described below in the case of such heat conduction, the temperature difference between the substrate and the hot plate is in proportion to the float distance. Thus, the local variation of the float distance of the substrate causes the temperature variation of the substrate.
The variation of the float distance is primarily caused by substrate deformation. The deformation of the semiconductor wafer increases through various processes, and the resulting shape is so complex that the shape cannot be estimated. Additionally, the deformation is unavoidably enlarged as the wafer size is increased. Therefore, it is suggested that the deformation should be corrected.
The process of putting the substrate in close contact to the hot plate as described in the aforementioned reference is one of the processes of correcting the deformation of substrates, but particles deposited on the back face of the substrate and the surface of the hot plate inevitably influence the process so that the variation cannot be routinely suppressed in a stable manner.
What has been described above is focused only on the heating system, but in a cooling system, a cooling plate is simply replaced for the hot plate, wherein the direction of heat conduction is opposite to the direction thereof in the case of the hot plate. Therefore, the advantages and problems are the same as described above. Hereinbelow, heating and cooling are collectively referred to as xe2x80x9ctemperature controlxe2x80x9d, and also hot plate and cooling plate are collectively referred to as xe2x80x9ctemperature control platexe2x80x9d.
It is an object of the present invention to overcome the problems of the prior art and provide a novel substrate temperature control system capable of unifying the temperature of the substrate and capable of shortening the temperature elevation time (temperature lowering time).
The problems of the present invention can be effectively solved by placing a temperature control plate (hot plate or cooling plate) with co-planar projections on the surface thereof as well as a substrate chuck mechanism fixing the substrate on the projections by pressing the substrate along the direction of the temperature control plate.
Through this arrangement of projections arranged, heat conduction through the contact surface to the projections and heat conduction through non-contact surface in a gas phase are formed between the substrate and the temperature control plate, but the heat conduction through the projections is prominent because the heat conduction through the projections is high compared with the conduction in gas. Therefore, the substrate temperature can be unified by unifying the heat transfer at the contact surface on the entire surface of the substrate. Because the efficiency of heat transfer then can be distinctively increased as compared with the conventional efficiency thereof through a gas phase, furthermore, the temperature elevation time (temperature lowering time) can be shortened. Because the float distance of the substrate is regulated and aligned through the co-planar projections, the deformation of the substrate is corrected so that the substrate becomes flat.
Additionally, a proposition to fix substrates by means of projections has been known as disclosed in Japanese Patent Laid-open No. Sho 62-45378. The system of the publication is a simple spin-coater for the purpose of separating a substrate from the gap for vacuum on a turn table thereby suppressing the temperature variation on the substrate, which variation develops in case that the substrate is put in close contact to a turn table with such gap. No reference is made therein about substrate heating (cooling) by utilizing temperature transfer from a temperature control plate, so that the system cannot suppress the temperature variation on a substrate to be heated.
In accordance with the present invention, furthermore, the efficiency of heat transfer is increased as the contact surface is larger, but particles deposited on the back face of a substrate and the surface of a hot plate influence the contact surface, with the resulting higher probability of temperature variation. Experimental results suggest that the upper limit of the ratio of the contact surface to the area of the whole back face is about 60%. Alternatively, the efficiency of heat transfer is decreased as the area of the contact surface is decreased, which induces the increase of the temperature elevation time (temperature lowering time). Experimental results suggest that the preferable lower limit of the ratio of the contact surface to the area of the whole back face is about 0.5%. Even in this case, the heat transfer through the contact surface is higher than the heat transfer through the non-contact surface. Based on experimental results, the preferable range is 20% to 50%, in particular.
The heat transfer through the contact surface between the substrate and the temperature control plate is controlled by means of thermal contact resistance. The heat quantity Q exchanged between the two can be represented by the formula (1), provided that contact surface is Sc and thermal contact resistance is Rc.
Q=Scxcex94T/Rcxe2x80x83xe2x80x83(1)
Provided that the temperature difference between the substrate and the temperature control plate is uniform at any point, herein, the heat quantity exchanged between the substrate and the temperature control plate per unit area is disproportional to the thermal contact resistance Rc. The variation of the resistance causes the variation of the heat quantity and the irregularity of the temperature distribution of the substrate. A formula calculating the thermal contact resistance Rc, generally known widely, is represented by the formula (2), provided that the roughness values of the contact surface of the substrate and the temperature control plate are xcex41 and xcex42, ( respectively; the thermal conductivities thereof are xcex1 and xcex2, respectively; the contact pressure is P; the thermal contact resistance is R0; Brinel hardness is H; and the thermal conductivity of the gas in the contact surface is xcexf.                               1                      R            c                          =                                            {                                                1                                                                                    δ                        1                                                                    λ                        1                                                              +                                          R                      0                                        +                                                                  δ                        2                                                                    λ                        2                                                                                            -                                                      λ                    f                                                                              δ                      1                                        +                                          δ                      2                                                                                  }                        ⁢                          P              H                                +                                    λ              f                                                      δ                1                            +                              δ                2                                                                        (        2        )            
The roughness values of the contact surface, namely xcex41 and xcex42, the thermal conductivities thereof, namely xcex1, xcex2 and xcexf, and the Brinel hardness H, are values intrinsic to a substance, while the thermal contact resistance R0 is empirically determined. Hence, the thermal contact resistance Rc is determined on the basis of the contact pressure P. Therefore, the vacuum pressure P is preferably controlled at a constant value, whereby the variation of the contact state between the back face of the substrate and the hot plate, which variation depends on the pressure variation, can be reduced, while the thermal contact resistance can be retained at a constant value. Through the thermal contact resistance at such constant value, the heat transfer at the contact surface can be unified on the whole surface of the substrate, whereby the substrate temperature can be unified.
For more detailed description of the contact surface, the contact surface between the projections and the back face of the substrate comprising countless micro-projections forming the surface roughness and microfine gaseous spaces embedded between these projections, forms a thermal contact resistance, which is expressed in the item including xcexf of the formula (2). When pressurizing along the direction of the temperature control plate of the substrate is done for example by vacuum chuck, the pressure of the gaseous space is decreased, but the gaseous thermal conductivity xcexf is almost equal to the conductivity at ambient pressure, with less variation of the thermal contact resistance if the pressure is about 10 Torr or more. If the pressure is below the value, the gaseous thermal conductivity is lowered to increase the thermal contact resistance and decrease the thermal conductivity.
Heat transfer at the non-contact surface in the gas phase is relatively slight, and description is now made concerning the heat quantity thereof. Heat transfer through the gas between the float distance (equal to the height of the projections in accordance with the present invention) of the substrate and the temperature control plate is via convection heat transfer and thermal conduction. If the float distance is slight, heat transfer is primarily done via heat conduction while the convection heat transfer is negligible. Provided that gaseous heat conductivity is xcex; the area of substrate at the non-contact surface not in contact to the projections is S; the temperature difference between the substrate and the temperature control plate is xcex94T; the float distance is h, the heat quantity QA exchanged between the substrate and the heat control plate via the gaseous heat conduction at the non-contact surface is represented by the formula (3) according to the Fourier law of heat conduction.
QA=xcexSxcex94T/hxe2x80x83xe2x80x83(3)
Thus, the temperature difference xcex94T is represented by the formula (4).
xcex94T=hQ/xcexSxe2x80x83xe2x80x83(4)
The formula (3) indicates that heat quantity QA is larger at the same temperature difference xcex94T, as float distance h is smaller. Because the formula (4) can be established locally at any position of the substrate face, it is indicated that the variation of the float distance h corresponds to the variation of the temperature difference xcex94T. In accordance with the present invention, the float distance h is aligned under the control of the co-planar projections, and therefore, the temperature variation based on the gaseous thermal conduction can be suppressed.
Because the thermal conduction through the projections is predominant as has been described above, the limitation of the height of the projections can greatly be reduced, compared with conventional cases wherein the thermal conduction through a gaseous phase is main. For vacuum chuck, however, the height above 1 mm readily causes disorders in the gaseous stream to be chucked; 1 xcexcm or below, the influence of particles as the problem of close contact readily occurs. Hence, the preferable range of the height of the projections is preset from 1 mm to 1 xcexcm.
As the press mechanism to pressurize and fix the substrate along the direction of the temperature control plate, a mechanism of for example vacuum chuck or electrostatic chuck may be employed. For using a vacuum chuck mechanism, a plurality of projections are arranged, together with a vacuum seal enclosing the projections and a vacuum chuck hole, on the faces of the temperature control plate. The vacuum seal on the same plane of the projections works as a vacuum seal so as to prevent the flow of air outside the vacuum seal into the space inside the vacuum seal. By chucking the space enclosed with these projections and the vacuum seals and the substrate into a negative pressure by using a hole for vacuum, a semiconductor wafer can be fixed at an equal distance at any position of the surface of the temperature control plate. Then, the contact surface for heat transfer be formed on the projections and the vacuum seals and the back face of the substrate.
For using an electrostatic chuck mechanism, a mechanism is arranged to load an electrostatic voltage between the substrate and an electrode, by embedding the electrode into a temperature control plate prepared from an insulator.
Because the pressure generated serves as the contact pressure in any case, the pressure of the space is preferably controlled to a predetermined constant value. It is indicated for vacuum chuck that the lower limit of the pressure is 10 Torr as described above, and the upper limit is 700 Torr, from the respect of the formation of contact resistance and the preparation of a substrate with flat surface, whereby preferable results are recovered.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as they become better understood by reference to the following detailed description when considered in connection with the accompanying drawings.